Magnetic apparatus for metal casting

ABSTRACT

Apparatus for metal casting using a magnetostatic field generator having at least one superconducting solenoid magnet disposed on at least one side of a mould or metal shell for setting up a heterogenious magnetostatic field of intensities at least in excess of 10,000 gauss in the liquid metal retained within the mould or metal shell to thereby agitate the liquid metal. The mageto-static field generator is accommodated within a cryostat, which is always held cool at a predetermined temperature by a special auxiliary cooling means.

United States Patent Sugazawa et al.

MAGNETIC APPARATUS FOR METAL CASTING Inventors: Kiyoshi Sugazawa; KiyotoUshijima,

both of Kobe; Kantaro Sasaki, Ashiya, all of Japan Assignees: SumitomoMetal Industries, Ltd.;

Sumitomo Shipbuilding and Machinery Co., Ltd., Japan Filed: Dec. 19,1973 Appl. No.: 425,951

Foreign Application Priority Data Dec. 20, 1972 Japan 47-128555 US. Cl164/147; l64/49 Int. Cl. B22D 27/02 Field of Search 164/49, 82, 146,I47, 250

[56] References Cited UNITED STATES PATENTS 6/1922 McNeil] 164/147 X9/1969 Utech et a1. 164/49 X Ill [ 1 Oct. 14,1975

3.809.l45 5/1974 Schafer l64/49 FOREIGN PATENTS OR APPLICATIONS 872,5917/1961 United Kingdom 164/49 Primary Examiner-R. Spencer AnnearAttorney, Agent, or Firm-Watson, Cole, Grindle & Watson ABSTRACTApparatus for metal casting using a magnetostatic field generator havingat least one superconducting solenoid magnet disposed on at least oneside of a mould or metal shell for setting up a heterogeniousmagnetostatic field of intensities at least in excess of 10,000 gauss inthe liquid metal retained within the mould or metal shell to therebyagitate the liquid metal.

The mageto-static field generator is accommodated within a cryostat,which is always held cool at a predetermined temperature by a specialauxiliary cooling means.

12 Claims, 8 Drawing Figures U.S. Patent Oct. 14, 1975 Sheet 2 of43,911,997

FIG. 3

, HWOQV/ FIG. 4

MAGNETIC APPARATUS FOR METAL CASTING This invention relates to metalcasting and, more particularly, to a process and apparatus for metalcasting, which makes it possible to prevent or reduce macro or micro ormicrometallographic heterogenity resulting chiefly at the center ofingots or continuously casted billets from the gradual cooling, coolingor rapid cooling of liquid metal poured into the mould or liquid metalcore remaining within the solidified metal shell. This is accomplishedby applying magneto-static field at least in excess of 10,000 gauss tothe liquid metal or liquid metal core during the solidification thereof,thereby to obtain an ingot or billet having a homogenious metallographicstructure.

It is well known in the art that when liquid metal is solidified, thecenter of the resultant body or casting center is prone to a sort ofheteroneious structure usually termed center porosity,center-segregation or inner-crack, this trend being particularlypronounced where the metal liquid is cooled in a still state throughrapid cooling, as is disclosed in Japanese Patent Publication No.33025/1972. It has also been known that this heterogenity may beeliminated or reduced only by agitating the liquid metal until thesolidification thereof, and there have been proposed various types ofmeans for causing the flow of the liquid metal by means ofelectromagnetic force.

The prior-art methods for electromagnetically causing the flow of liquidmetal include: a rotating electromagnetic field method as disclosed, forinstance, in Dukefriet Unkhans patent (Japanese Patent Publication No.9962/1956); one using an electromagnetic field set up by three-phasealternating current as disclosed in Japanese Patent Publication No.32486/1972; and one using a travelling electromagnetic field set up byalternating current as disclosed in US. Pat. No. 3,656,537. All theseprior-art methods have resorted to a copper wire coil which is disposedto surround or disposed in the close proximity of the mold or metalshell and through which single or three phase alternating current iscaused, whereby the liquid metal is electromagnetically agitated due toalternating magnetic field set up in it and eddy current induced in it.

In the above prior-art method, which use alternating current forobtaining the agitating force, very large current has to be passedthrough the copper wire coil. Therefore, the ampere turns of the copperwire coil has an extremely large value, so that water cooling or oilcooling is necessary to remove the Joules heat. Also, the magnetic fieldgenerator coil as a whole is very large in size. Particularly, where thecoil surrounds a large casting body, its operability will be limited dueto the .loules heat, and its cost as a practical unit will be enormous.

According to the invention, unlike the aforementioned prior-arttechniques having resort to alternating magnetic field for obtainingstrong agitating force, a very intensive magnetic-static field at leastin excess of 10,000 gauss is set up in the liquid metal by a singlemagnet.

Thus, the size and cost of the magnetic field generator may be extremelyreduced. While sufficient electromagnetic agitating effect can beobtained with a single magnetic field generator or magnet according tothe invention, further strong electromagnetic agitating effect may begiven to the liquid metal by a combination of a plurality ofmagneto-static field generators arranged such as to provide the mosteffective magnetic flux distribution. Thus, it is possible to obtain astrong agitating force that could not be obtained by the priorartalternating current method with an extremely small and inexpensiveapparatus as compared to the prior-art one.

In accordance with the invention, the magneto-static field is set up bydirect current through a superconducting solenoid magnet. Thesuperconducting solenoid is made of a superconducting metal having acharacter of offering zero electric resistance at the temperature ofliquid helium (268.9C) such as a niobium-tin intermetallic compound anda niobium-titanium alloy, and it is held at the termperature of liquidhelium when it carries direct current. Since the solenoid offers zeroelectric resistance at the working temperature, it may use a very finewire or filament and may be formed in a very small size with a largenumber of turns. Also, since the solenoid as a whole may be readily heldat the liquid helium temperature, it is readily possible to produce ahigh magneto-static field of 70,000 to 100,000 gauss at the center ofthe solenoid.

The above and other objects, features and advantages of the presentinvention become more apparent from the following description inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a set-up using superconductingsolenoid magnets according to the invention applied to a still castingprocess;

FIGS. 2 to 4 are schematic representations of set-ups according to theinvention applied to a continuous cast-- ing process;

FIG. 5 shows a cryostat;

FIG. 6 shows a schematic liquid helium circulating system used for acontinuous casting process according to the invention; and

FIGS. 7A and 7B are graphs showing sulphur and carbon assay contentdistributions in a billet obtained in accordance with the invention anda billet obtained without using any agitating electromagnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a set-up accordingto the invention with a superconducting solenoid magnet disposed in thevicinity of one side of a still mold for setting up an intensivemagneto-static field in the liquid metal within the mold. In FIG. 1reference numeral 1 designates a mould made from a non-magneticmaterial, and numeral 2 a solid metal shell surrounding liquid metal 3.Numeral 5 designates the superconducting solenoid magnet, which isdisposed in the vicinity of one side of the mold that the magnetic fluxgenerated by it may act upon the liquid metal. The solenoid magnet 5 issupported within a cryostat 7, and in this embodiment it is held inposition by a non-magnetic support 8. The cryostat 7 is a highperformance insulated vessel having an evacuated double-wall insulatingstructure made of a non-magnetic metal. It is filled with liquid helium,whose surface level 9 is held at a substantially constant level. Theliquid helium is replenished from a feed pipe 10, and evaporated heliumis recirculated through a discharge pipe 1 l to a helium liquifier. Thecryostat has an upper flange 21 provided with a power lead terminal 12for supplying power to the solenoid magnet 5. A heat insulator 15 isinsterposed between the cryostat and the mold for protecting the wallstructure of the cryostat. The magnetic flux 6 set up by the solenoidmagnet and acting upon the liquid metal may be moved relative to theliquid metal by vertically oscillating the cryostat by means of avertically movable stand 13 or by horizontally moving the mould byvertically movable stand 13 or by horizontally moving the mould by meansof wheels 16 and rails 17, so that the intensive magneto-static fieldmay efficiently act upon the liquid metal so as to evoke the fiowthereof in the desired direction. Of course, it is desirous to this endto move both the cryostat and the mold simultaneously. Also, it ispossible to move the cryostat in the horizontal direction and the moldin the vertical direction.

FIGS. 2 to 4 show other forms of the invention applied to the continuouscasting process. In the Figures, the cryostat is not shown but isimplied. In FIGS. 2 and 3, numeral 1 designates a bottomless watercooled mold, and numeral 2 a solidified metal shell enclosingnon-solidified or liquid metal 3. The billet containing the liquid metalis withdrawn at a substantially constant speed in the direction of arrow4. In both these arrange ments, two solenoid magnets are provided withrespectively opposite poles (N and S poles) directed toward the shell.In the example of FIG. 2, both the solenoid magnets and 5-1 are disposedon the same side of the shell and at different vertical positions. Withthis arrangement, it is possible to vary the magnetic field intensity ormagnetic flux density by varying the distance between the two magnets.In the example of FIG. 3, the two solenoid magnets 5 and 5-1 aredisposed on opposite sides of the shell and at different verticalpositions. With the disposition of the solenoid magnets with respect tothe lateral direction of the billet as in the example of FIG. 2, anextremely improved agitating effect may be obtained.

In case of the continuous casting, the billet is withdrawn substantiallyat a constant speed, and this means that the solenoid magnet is alwaysmoved relative to the liquid metal. Thus, in this case the cryostatoscillating means as has been mentioned in connection with FIG. 1 is notneeded. The agitating effect of the magneto-static field upon the liquidmetal is attributable not only to the eddy current induced but also tothe gradient of the intensity or flux density of the field in the liquidmetal. In other words, the liquid metal about to solidify is subjectedto the combined effect of eddy current and gradient of the field. Infurther detail, in case of FIG. 3 example, for instance, the billet iscontinuously withdrawn, so that the liquid metal enclosed within thesolid shell proceeds usually at a speed of, for instance, 0.5 to 3meters per minute. Due to its interaction with the magneto-static field,the liquid metal experiences forces tending to stop its movement.However, since there is a gradient of the field in the liquid metal,there also results a gradient of the intensity of the force acting uponthe liquid metal, so that the liquid metal is agitated. Although agradient of the field is formed solely with a single magnet, themagnetic flux pattern may be changed to suit the type of casting andvarious specifications of the ingot or billet to be produced byappropriately changing the number and disposition of the solenoidmagnets as typically shown in the examples of FIGS. 2 to 4.

Also, the solenoid magnet for generating the magnetic field is designedby taking various conditions such as the intensity of the exerted forceand the position of installation into consideration.

FIGS. 5 and 6 show examples of apparatus required for the execution ofthe invention. FIG. 5 shows an example of the cryostat. The illustratedcryostat, generally designated at 7, essentially consists of an upperpart having a lower flange 22 and a lower part accommodating a solenoidmagnet 5. Its interior is in comm unication with a vacuum pump notshown, through a pipe 23 and is held under a pressure of [0'5 mm Hg.Numeral 24 designates a magnet case, whose top communicates with a pipe25. Liquid helium is supplied from a liquid helium feed pipe 10 to fillthe case 24 and pipe 25 to a constant level 9. Evaporated helium isrecirculated through a discharge pipe 11 to a helium liquifier as shownin FIG. 6. This example uses liquid nitrogen having a vaporizationtemperature of -l 958C as auxiliary cooling means to take up externalheat with respect to the cryostat for ensuring steady cooling effect ofthe liquid helium. More particularly, a liquid nitrogen chamber 26having a plurality of downwardly extending fine tubes 27 is providedwithin the upper part of the cryostat. Liquid nitrogen is suppliedthrough a supply tube 29, and evaporated nitrogen is exhausted throughan exhaust tube to the out side. The cryostat used for the invention isentirely made of such non-magnetic material as stainless steel 304.Although not shown, the actual apparatus will include a liquid levelgauge, a thermometer, a vacuum gauge, power lead terminals for themagnet 5, a magnet position adjuster, a safety device and heatinsulation means.

FIG. 6 shows an example of the liquid helium supply system employed fora continuous casting process of curved strand type. In this process,liquid metal is poured from a pouring system 31 into a bottomless watercooled mold 1. Solenoid magnets 5 and 5-2 are arranged in a way as inthe example of FIG. 3 on the path of the strand 2 emerging from the mold1 directly below or in the secondary cooling zone. Liquid helium andliquid nitrogen are supplied to the cryostat and evaporated gas isrecovered in the manner as described above. Only the recirculation ofhelium is shown. Helium gas stored under a pressure of about atm. inhelium gas bombs 33 and under a pressure of about I atm. in a helium gastank 34 is supplied to a helium gas compressor 35, and pressurisedhelium gas from the compressor 35 is supplied to a high pressure heliumgas tank 36, and thence to a helium liquifier 37. In the heliumliquifier, the high pressure helium gas is subjected to heat exchangewith liquid nitrogen and then passed through a gas expander, whereby itis rendered into liquid helium, which is supplied through a liquidhelium tank 38 to the cryostat 7. Also, evaporated helium from thecryostat goes to the helium gas tank 34 either through the liquifier 37or directly for recovery of liquid helium for recirculation. Althoughliquid helium is expensive, substantially no helium is lost in thecourse of the recirculation involving evaporation and liquefaction sincethe processed helium is recirculated through a closed loop. Also, in theiron making plants provided with apparatus for producing oxygen, liquidnitrogen is inexpensively and readily avialable as a by-product. Thus,no debit factor is found from the standpoint of the running cost.

The following experimental example demonstrates the effects attainableaccording to -the invention.

EXPERIMENT A quantity of 2,000 kilograms of molten low carbon steel,which was produced by a high frequency induction furnace, was castedwith an experimental vertical continuous casting apparatus provided witha magneto static field generator according to the invention. Thechemical composition of the steel was 0.15 percent carbon, 0.30 percentsilicon, 0.71 percent manganese, 0.010 percent phosphor, 0.012 percentsulphur and 0.030 percent soluble aluminum, the rest being iron. Theresultant billet had a thickness of 130 millimeters and a width of 260millimeters. The magneto-static field generator was disposed directlybelow the mold, with its magnets positioned in the proximity of onebroader side of the billet, that is as in the arrangement in FIG. 2. Thefirst half of the steel, namely 1,000 kilograms of steel, was castedwithout applying any magnetic field, and the field was applied for theremaining half. The billet obtained in this way, was sampled at itspositions corresponding to the feed of 400 kilograms and 1,600 kilogramsrespectively from the start of the casting. From each of these samplesassay test pieces were cut out at intervals of 2 to millimeters in thedirection ofthe thickness of the billet for examining the segregation ofcomponent elements. The results of the tests are shown in FIGS. 7A and78. It will be seen that the billet obtained by applying themagneto-static field resulted in very little segregation of carbonsulphur at the center compared to the billet obtained without any fieldapplied. Also, it was confirmed that the magnetostatic field applied isparticularly effective in the refinement of the dendrite grain structureand capable of reducing the segregation coefficient down to well below2.0. The specifications of the magnets used in this experiment were asfollows:

Number of magnets: 2 Type and size of the magnets: Solenoid type magnetwith an inner diameter of 50 mm, an outer diameter of 180 mm and a widthof 50 mm Number of turns of the solenoid: 22,000

Solenoid wire: Copper wire 0.4 mm in diameter and containing 40 sealedfine filaments of Nb-Ti alloy resin coating being provided after windingthe wire.

Current: 30 amperes Field intensity: 70,000 gauss at the center ofsolenoid and 22,000 gauss at the end of solenoid Initial liquid heliumconsumption: 25 liters As has been shown, by using the superconductingsolenoid magnet, a magneto-static field of high flux density, namely inexcess of 10,000 gauss which could never be attained by passing largecurrent through the conventional solenoid coil, may be obtained in alarge space inexpensively and with a very small unit to effectivelyexert an agitating force to the liquid metal. Also, as has been verifiedby experiments, it is possible to suppress segregation andcenter-porosity and reduce the segregation coefficient down to 2.0 orbelow. Further, in addition to the fact that the growth of dendrites canbe repressed by the slight agitation of the metal liquid, crystallineprecipitates or ionized concentrates of higher susceptibility than theoriginal liquid metal are affected by the very intense magnetic fieldand tend to be attracted toward the higher flux density. While the aboveexperimental results are obtained by using the experimental continuouscasting apparatus, it may be readily understood that similar results maybe obtained in the case of the set-up of FIG. 1 provided the mold andmagnet are moved relative to each other, and application to large ingotsis possible. (In this case, the mold should be made of a non-magneticmaterial such as stainless steel.)

From the above grounds, it is possible to rapidly cool the liquid metalwithout resulting in defects attributable to the central heterogeniousstructure. Thus, it is possible to adopt the rapid cooling system so asto increase the billet withdrawal speed and hence reduce the possibilityof crack formation in the rolling process, so that the invention willgreatly contribute to the betterment of the yield of products.

What is claimed:

1. An apparatus for casting metals comprising means for holdingnon-solidified metal, means for generating a heterogeniousmagneto-static field in said nonsolidified metal, said generating meansbeing disposed in the vicinity of one side of said holding means, andmeans for causing relative movement of said holding means and generatingmeans to each other.

2. The apparatus according to claim 1, wherein said magneto-static fieldgenerating means includes at least one super-conducting solenoid magnet.

3. The apparatus according to claim 2, wherein said solenoid magnet ishollow and formed by winding a wire containing very finesuper-conducting filaments sealed therein and has a resin coatingprovided after the winding of the wire.

4. The apparatus according to claim 1, wherein said holding means is amold.

5. The apparatus according to claim 4, wherein said moving meansincludes means for vertically move said generating means.

6. The apparatus according to claim 4, wherein said moving meansincludes wheels carried by a wagon supporting said mold and rails toguide and support said wheels.

7. The apparatus according to 'claim 1, wherein said holding means is ashell formed as a result of solidification of said non-solidified metal.

8. The apparatus according to claim 7, wherein said moving means is amechanism for continuously withdrawing said shell.

9. The apparatus according to claim 8, wherein the speed of withdrawalof said shell ranges from 0.5 to 3 meters per minute.

10. The apparatus according to claim I, wherein said magneto-staticfield generating means can generate a magneto'static field ofintensities above 10,000 gauss in said non-solidified metal.

11. The apparatus according to claim 1, wherein said magneto-staticmeans includes a plurality of superconducting solenoid magnets disposedon opposite sides of said holding means with opposite poles directedtoward the opposite sides of said holding means.

12. The apparatus according to claim 1, wherein said magneto-staticmeans includes a plurality of superconducting solenoid magnets disposedon opposite sides of said holding means with like poles directed towardthe opposite sides of said holding means.

1. An apparatus for casting metals comprising means for holdingnon-solidified metal, means for generating a heterogeniousmagneto-static field in said non-solidified metal, said generating meansbeing disposed in the vicinity of one side of said holding means, andmeans for causing relative movement of said holding means and generatingmeans to each other.
 2. The apparatus according to claim 1, wherein saidmagneto-static field generating means includes at least onesuper-conducting solenoid magnet.
 3. The apparatus according to cLaim 2,wherein said solenoid magnet is hollow and formed by winding a wirecontaining very fine super-conducting filaments sealed therein and has aresin coating provided after the winding of the wire.
 4. The apparatusaccording to claim 1, wherein said holding means is a mold.
 5. Theapparatus according to claim 4, wherein said moving means includes meansfor vertically move said generating means.
 6. The apparatus according toclaim 4, wherein said moving means includes wheels carried by a wagonsupporting said mold and rails to guide and support said wheels.
 7. Theapparatus according to claim 1, wherein said holding means is a shellformed as a result of solidification of said non-solidified metal. 8.The apparatus according to claim 7, wherein said moving means is amechanism for continuously withdrawing said shell.
 9. The apparatusaccording to claim 8, wherein the speed of withdrawal of said shellranges from 0.5 to 3 meters per minute.
 10. The apparatus according toclaim 1, wherein said magneto-static field generating means can generatea magneto-static field of intensities above 10,000 gauss in saidnon-solidified metal.
 11. The apparatus according to claim 1, whereinsaid magneto-static means includes a plurality of super-conductingsolenoid magnets disposed on opposite sides of said holding means withopposite poles directed toward the opposite sides of said holding means.12. The apparatus according to claim 1, wherein said magneto-staticmeans includes a plurality of super-conducting solenoid magnets disposedon opposite sides of said holding means with like poles directed towardthe opposite sides of said holding means.